Several cardiac parameters of clinical interest for detecting or monitoring important health conditions can only be determined from measurements of moment-by-moment cardiac chamber volumes and cardiac blood flows. For many health conditions, it is desirable that these parameters be determined from time-to-time with only minimal inconvenience and burden to a subject patient.
Several methods are known in the prior art for measuring volumes and flows many of which are inconvenient and burdensome to varying degrees. For example, volumes and flows can be determined from invasive clinical procedures, e.g., cardiac catheterization, that are best performed in operating-room-like conditions, are highly inconvenient and burdensome, can be performed only rarely, and require highly trained medical personnel. Volumes and flows can also be determined by less burdensome, non-invasive procedures, e.g., ultrasound examination, cardiac kymography (CKG, also known as displacement cardiography), and the like. However, ultrasound examination usually requires trained medical personnel and is commonly requires that the patient be at rest. CKG provides a usable output signal only when its electromagnetic field electrode has been carefully positioned near the patient's chest; the signal is lost upon any movement of the electrode.
A further non-invasive method for extracting moment-by-moment cardiac chamber volumes and cardiac blood flows is known as thoracocardiography (TCG). TCG measures the mechanical displacement of the anterior chest wall overlying the heart, and extracts from an input chest wall motion signal an output signal reflecting primarily cardiac motion. Although TCG measurements (depending on the sensor used) are at most minimally inconvenient and burdensome and do not require trained medical personnel, prior art methods and systems have had difficulty in extracting clinically useful cardiac signals. This is primarily because cardiac activity makes a very small contribution to total chest wall motion, and further because the frequencies of cardiac activity overlap the frequencies of other much larger signal components.
In more detail, the total chest wall motion signal includes components arising from motion due to respiration, from motion due to cardiac expansion and contraction, and from artifact motions due to, e.g., subject motion or equipment noise. The cardiac component has an amplitude that is at most about 1-4% of the amplitude of the respiratory component. Further, its frequency usually varies between about between about 0.8 Hz and 1.7 Hz (while resting but higher during activity) with harmonics both above and below this range, and therefore overlaps the frequency of the respiratory component which usually varies between about 0.2 Hz and about 0.5 Hz with both higher and lower harmonics. The artifact component can have unpredictable amplitude and a widely-varying frequency.
Therefore, there remains a need for less inconvenient and burdensome method for measuring moment-by-moment cardiac chamber volumes and cardiac blood flows that provides clinically useful cardiac signals.